Pregnancy Due Date Calculator
Enter whatever you actually know — last period, ovulation test date, when you conceived, an early ultrasound, or your IVF transfer date — and this calculator estimates your due date with each method side by side, so you can see how much the answers really differ.
Your details
If your anchors disagree by more than 7 days, an early dating ultrasound (before 14 weeks) is the standard way to settle the question — ask your midwife or OB at your first appointment.
Most due date calculators ask for one number — the first day of your last period — and quietly assume your cycle is exactly 28 days long. That assumption is wrong for roughly half of women. If your cycle runs 33 days, a standard calculator places your due date about five days too early, which matters most in the first trimester, when the screening windows for nuchal translucency (11w0d–13w6d) are narrow and unforgiving.
This calculator works differently. Every anchor you provide gets its own due date estimate: the classic Naegele rule (last period plus 280 days), a cycle-length-corrected version of it, your ovulation or conception date plus 266 days, and early-ultrasound dating. If two anchors disagree by a week or more, you will see that spread on screen instead of getting a single overconfident number. An optional advanced mode adds the Mittendorf-Williams adjusted estimate, a research rule that accounts for whether this is your first baby, your age, BMI, and background.
The result is not one due date but a range with a stated spread — a more honest answer, and the one that matches how obstetricians actually date a pregnancy when the information is ambiguous.
How the calculation works
The Naegele rule adds 280 days (40 weeks) to the first day of your last period. It assumes you ovulated on day 14 of a 28-day cycle. The cycle-corrected anchor instead computes your likely ovulation day (last period plus cycle length minus 14) and adds 266 days — the true biological length of pregnancy from conception.
Ovulation and conception dates add 266 days directly. Ultrasound dating reverses the usual logic: if a sonographer measured you at 8 weeks 0 days on the scan day, the due date is scan day plus (280 − 56) days.
The Mittendorf-Williams mode applies corrections from a 1990 study of 3,600+ spontaneous pregnancies: first-time mothers averaged about 8 days longer than 280, women who had given birth before averaged about 3 days longer, with small adjustments for age over 30 and BMI. Because later, larger studies only partly reproduced these effects, we show it as a separate line rather than blending it in.
Frequently asked questions
How accurate is a due date calculator?
A due date from a known conception or IVF transfer date is accurate to within a few days. A due date from last period alone is typically within ±1–2 weeks, because cycle length and ovulation day vary. Only about 4–5% of babies arrive on their estimated due date; the EDD is best understood as the center of a five-week window (37–42 weeks) that is considered normal.
Why do different calculators give me different due dates?
Because they use different anchors. A calculator that assumes a 28-day cycle will date you earlier than your real ovulation day implies if your cycle is longer than 28 days — and vice versa. This calculator shows every anchor side by side so you can see exactly where the disagreement comes from.
My LMP date is uncertain because of implantation bleeding. What should I enter?
Implantation bleeding (light spotting 6–12 days after ovulation) is often mistaken for a light period, and entering it as your LMP can shift your whole timeline by two weeks or more. If that might be you, rely on the ovulation/conception or ultrasound anchors instead, and mention the uncertainty to your clinician — the dating ultrasound before 14 weeks exists precisely for this situation.
What is the Mittendorf-Williams rule and should I trust it?
It is a 1990 research finding that uncomplicated pregnancies last longer on average than the textbook 280 days — 288 days for first-time mothers in their study group. Some later studies reproduced a smaller effect, others found none. We include it because "am I actually overdue?" is a real and stressful question, but the adjusted date is a reference point for discussion, not a reason to decline monitoring.
When is a due date considered late enough to induce?
Most guidelines recommend offering induction by 41–42 weeks, because stillbirth risk rises gradually after 41 weeks. Before 39 weeks, induction is generally reserved for medical reasons. Your own risk factors — age, blood pressure, gestational diabetes — can move these thresholds, so this is a conversation, not a calculator output.
References & sources
- Mittendorf R, et al. The length of uncomplicated human gestation. Obstet Gynecol. 1990.
- ACOG Committee Opinion 700: Methods for Estimating the Due Date (2017, reaffirmed).
- Hadlock FP, et al. Fetal age assessment using ultrasound. AJR Am J Roentgenol. 1984.
- Naegele FR. Lehrbuch der Geburtshülfe (1830) — origin of the 280-day rule.
- WHO recommendations on antenatal care (2016) — ultrasound dating before 24 weeks.